This invention relates to a method of detecting abnormality in a system for detecting the concentration of an ingredient in the exhaust gases, including a sensor for detecting the same concentration, in a fuel supply control system of an internal combustion engine which is adapted to perform feedback control of the air-fuel ratio of an air-fuel mixture being supplied to the engine in response to an output signal from the sensor.
In order to control the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine to a value within a desired range, a method is already known which is adapted to detect the concentration of a particular ingredient contained in exhaust gases emitted from the engine, e.g. the concentration of oxygen, determine the value of a correction coefficient for the air-fuel ratio in response to a detected value of the oxygen concentration, and correct the value of the air-fuel ratio by the use of the thus determined air-fuel ratio correction coefficient so that the value of the air-fuel ratio falls within the desired range.
An oxygen (O.sub.2) sensor is widely employed as the means for detecting the oxygen concentration, which is composed of, for instance, solid electrolyte of zirconium (ZrO.sub.2). This type O.sub.2 sensor has such a characteristic that its electromotive force abruptly changes when the air-fuel ratio of the mixture lies in the vicinity of the theoretical mixture ratio. More specifically, it assumes a high level when the air-fuel ratio is richer (smaller) than the theoretical mixture ratio, and a low level when the air-fuel ratio is leaner (larger) than the theoretical mixture ratio. However, if an abnormality occurs in the system for detecting the exhaust gas ingredient concentration including the O.sub.2 sensor having such characteristic, due to a disconnection in the wiring, degradation in the performance of the O.sub.2 sensor per se, etc., it will be impossible to accurately control the air-fuel ratio of the mixture being supplied to the engine. Therefore, it is necessary to always monitor the operation of the O.sub.2 sensor in order to obtain normal operation of the system for detecting the exhaust gas ingredient concentration.
A conventional method of detecting an abnormality in the system for detecting the exhaust gas ingredient concentration is known from Japanese Provisional Patent Publication (Kokai) No. 58-222939, as shown in FIG. 1, which shows a manner in which the value of the air-fuel ratio correction coefficient KO.sub.2 is varied, which is set to a value obtained by adding thereto or subtracting therefrom a predetermined value each time the output voltage value of the O.sub.2 sensor traverses a reference voltage value which corresponds to the desired air/fuel ratio (proportional term control), and thereafter it is set to a value obtained by adding thereto or subtracting therefrom a small fixed value each time a predetermined period of time elapses until the output value of the O.sub.2 sensor is inverted again (integral term control).
According to this conventional abnormality-detecting method, the time interval is detected at which the value of the correction coefficient KO.sub.2 is varied in a stepwise manner, i.e. the time interval (T1, T2, . . . T5 in FIG. 1) at which it is inverted from a value to make the air-fuel ratio richer to a value to make the air-fuel ratio leaner, or vice versa. It is determined that the system is operating abnormally if the detected time interval exceeds a predetermined period of time TFS (for example, if the time interval T5 from t5 to t6 is larger than TFS). And the value of the correction coefficient KO.sub.2 is set to a predetermined value at the fault detection (t6 in FIG. 1), thereby executing compensation for the abnormality in the system.
Another abnormality detecting method is known from Japanese Provisional Patent Publication (Kokai) No. 59-3137, which comprises detecting whether or not the value of the correction coefficient KO.sub.2 falls within a range defined by an upper limit value KO.sub.2 H and an lower limit value KO.sub.2 L thereof, that can be assumed during normal operation of the engine, measuring the period of time which has elapsed from the time the value of the correction coefficient KO.sub.2 fell outside the range, and determining that the system for detecting the O.sub.2 concentration is abnormal if the measured period of time exceeds a predetermined period of time TFS'.
However, although these known methods are capable of detecting abnormalities resulting in a distinct change in the output characteristic of the O.sub.2 sensor, such as caused by a disconnection in the wiring, they cannot detect abnormalities resulting in a gradual change in the sensor output characteristic. To be specific, let is now be assumed that values of the correction coefficient KO.sub.2 obtained during a period B in FIG. 1 have been obtained under the same operating condition of the engine as those obtained during a preceding period A in FIG. 1, and a mean value KREF2 of the values of the correction coefficient KO.sub.2 obtained during the period B is located on a richer side than a mean value KREF1 of the values of same obtained during the period A, so as to make the air-fuel ratio richer. If such phenomenon has actually been caused by a change in the output characteristic of the O.sub.2 sensor due to degradation in the performance thereof, such change can badly affect the emission characteristic and fuel consumption of the engine. Therefore, such degradation in the performance of the O.sub.2 sensor should desirably be detected as early as possible. However, according to the above conventional methods, abnormalities in the O.sub.2 sensor cannot be detected until the output value of same falls outside its normal range, or until the time interval at which the output value of same has been inverted with respect to a predetermined value exceeds a predetermined period of time.